For years, experts have pointed to poor diet and a lack of exercise as the key contributors to obesity. Professor Gale Carey said there could be a third, completely invisible factor.

"The average person has maybe 300 different, synthetic chemicals in their body that weren't meant to be there," Carey said.

The chemicals are in tables and chairs, couches, home electronics and carpet padding. To those common household items, manufacturers added chemicals called polybrominated diphenyl ethers, or PBDEs.

"They've been used as flame retardants -- so they're well-intentioned -- and incorporated into consumer products that could catch fire," Carey said.

Introduced in the 1970s, PBDEs are everywhere, Carey said.

"And so as these products break down or as they're put into landfills, these things are coming out into the environment," she said. "They get into the air. We know they get into the water, to the soil. They make their way into food."

In research conducted with her students, Carey determined that when it comes to human metabolism, PBDEs fit in like a perfectly formed puzzle piece.

"We've found that the chemical in the liver interferes with the activity of an enzyme that plays a key role in fat and glucose metabolism in the liver," she said.

Carey said the interference diminishes the enzyme's activity by 50 percent. That particular enzyme forms the backbone of the process that helps metabolize fat.

"The reason that concerned us is if the fatty acids can't be attached to the backbone, that has been shown to be related to insulin resistance, which is what type-2 diabetes is," she said.

Doctors said diagnoses of type-2 diabetes have gone up dramatically in the past 50 years.

Carey said it's not clear how much of a role PBDEs may be playing in making more Americans overweight, but her research shows there is a link.

"These chemicals are in consumer products, get out of the products and into our bodies," she said. "They're not meant to be in our bodies. That wasn't the intention of putting them into the products, but they are."

Carey said many companies stopped adding PBDEs to their products in the past decade, but that action was entirely voluntary. She said it is still a struggle to find an item such as a mattress that is completely free of the potentially harmful compounds.